The present invention is related to Japanese patent application No. Hei. 11-373538, filed Dec. 28, 1999; 2000-314990, filed Oct. 16, 2000, the contents of which are incorporated herein by reference.
The present invention relates to a fuel injection pump for an internal combustion engine, and more particularly, to a fuel injection pump having increased strength characteristics.
In conventional fuel injection pumps, a movable member, such as a plunger, reciprocates in a pump housing cylinder to pressurize fuel. This fuel is pressurized in a fuel pressurizing chamber, defined within the cylinder, between the pump housing and the plunger. The fuel is sucked into the pressurizing chamber from a fuel intake passage as the plunger slides downward in the cylinder. The fuel is then discharged from the pressurizing chamber into a fuel discharge passage as the plunger slides upward in the cylinder, thereby pressurizing the fuel. The discharged high pressure fuel is thereafter supplied to and accumulated in a common rail.
In a known diesel engine having this common rail fuel system, the fuel is pressurized to approximately 180 MPa by the fuel injection pump. Stresses induced by the relatively high fuel pressure are mainly concentrated in corners of the pressurizing chamber.
During fuel pressurization, an inner cylindrical wall surface of the pressurizing chamber as well as an inner cylindrical wall surface of the fuel intake passage and of the fuel discharge passage (these passages are collectively referred as a fuel passage) are subjected to a radial pressure force due to high pressure fuel therein. Therefore, a connecting segment between the inner wall surface of the pressurizing chamber and the inner wall surface of the fuel intake passage as well as a connecting segment between the inner wall surface of the pressurizing chamber and the inner wall surface of the fuel discharge passage experience large tensile force.
Since these connecting segments are located in the cylindrical inner wall surface of the pressurizing chamber, stress applied to each connecting segment due to tensile force is mainly concentrated at intersecting points (axial intersecting points) between the connecting segment and a longitudinal central axis of the connecting segment that extends through a center of the connecting segment in an axial direction with respect to the pressurizing chamber.
To provide a structure capable of withstanding such a stress, the wall of the pump housing around the cylinder can be thickened or can be made of a stronger material. However, an increase in the wall thickness of the pump housing generally causes an increase in the size of the fuel injection pump. Furthermore, the use of the stronger material, which is usually more expensive, generally causes an increase in the manufacturing cost of the fuel injection pump.
Alternatively, the connecting segments may be chamfered to spread the stresses. In order to chamfer the connecting segments, a small diameter electrode needs to be placed adjacent to the connecting segment to permit electric discharge between the electrode and the connecting segment to remove material from the connecting segment. Alternatively, a slurry containing an abrasive agent needs to be passed through the fuel intake passage and the fuel discharge passage to polish the corners within these passages and connecting segments. However, in the electric discharge machining process, since the electrode and other processing tools have very small diameters, very precise control of these tools is required. Also, the polishing process with the slurry is time consuming and tedious.
Furthermore, higher fuel injection pressures (for example, 200 MPa or more) are required to meet more stringent future emission regulations. In such a case, the described processes cannot provide adequate connecting segments able to withstand the stresses induced by the higher fuel injection pressures.
The present invention provides a fuel injection pump having improved strength and using higher fuel injection pressures without increasing the size of the fuel injection pump. Here, a stress spreading means is provided at a connecting segment between a first cylindrical inner surface, defining a fuel pressurizing chamber, and a second cylindrical inner surface, defining a fuel passage in a pump housing. The stress spreading means is arranged to contain the axial intersecting points of the connecting segment. The stress spreading means spreads the stresses concentrated in the axial intersecting points over adjacent regions of the axial intersecting points. Therefore, it is not necessary to increase the wall thickness of the cylinder or to use a stronger material for the cylinder wall to improve the strength of the connecting segment. As a result, the strength of the fuel injection pump can be effectively improved at low cost without increasing the size of the fuel injection pump, allowing the use of higher fuel injection pressures.
In another aspect of the present invention, the stress spreading means is a flat section that extends perpendicular to an axis of the fuel passage. Since the stress spreading means is flat, more of the connecting segment is located in a single plane compared to where the connecting segment is located in the cylindrical surface. Therefore, the stresses concentrated in the axial intersecting points can be spread over the entire connecting segment located in the flat section, alleviating the stress concentration at the axial intersecting points. As a result, the strength of the fuel injection pump can be improved with by using a simple structure.
In another aspect of the present invention, the stress spreading means is a curved section having a radius of curvature that is greater than the inner wall surface of the pressurizing chamber. By making the curved section radius of curvature larger than the first cylindrical inner surface that defines the pressurizing chamber, the connecting segment is located in the less curved surface. Therefore, the stresses concentrated in the axial intersecting points can be spread over the less curved connecting segment, alleviating the stress concentration on the axial intersecting points. As a result, the strength of the fuel injection pump can be improved with this simple structure.
In another aspect, the stress spreading means contains the entire periphery of the connecting segment. Therefore, the stresses concentrated in the axial intersecting points is spread over the entire periphery of the connecting segment. As a result, the strength of the fuel injection pump can be improved.
While the above-described embodiments refer to examples of usage of the present invention, it is understood that the present invention may be applied to other usage, modifications and variations of the same, and is not limited to the disclosure provided herein.